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Colour Assessment of Prepainted Metal

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Colour Assessment of Prepainted Metal Colour assessment of prepainted metal ECCA guidelines Why is colour management important? The directionality of metallics should be consistent especially when panels are cut to size To ensure tonal consistency, all material should come from the same production batch To ensure tonal consistency, all material should come from the same production batch If different materials are used nextColour to each other in (e.g.practice – what does it powder coated and prepainted),mean colourfor the matching end user / client? of the powder coated components should be done to an actual sample of the cladding material – not to a theoretical value or a RAL reference. Colour: basic principles Basic principles Colour • Colour stimulus consists of three components • The light source sending out light waves (electromagnetic radiation) and the surrounding conditions (environment around the light source and the object) • The observed object that reflects a part of the light waves • The observer that detects/the eyes that absorb the light waves reflected on the surface of the object Basic principles • The light source • The light source emits electromagnetic radiation at different wavelengths that all relate to a different colour. All visible light is a mixture of wavelengths, each corresponding to a different colour, which is specific to a light source. Therefore, a colour of an object can seem to vary under different light sources. • The object • Any object reflects certain wavelengths of the incoming light and absorbs the others. Therefore, an object that reflects green light waves and absorbs the others looks green. Other aspects that affect the colour observation are shape and surface. The object can be embossed or textured, or have metallic, pearlescent or phosphorescent effects that all influence our experience of colour. • The observer • The defining observer of colour is the human eye. Thus, the colour experience is highly subjective. The ability of the human eye and brain to detect and handle colours is individual and depends on age, gender, inherited traits, and even mood. Colour characterisation Colour characterisation What do we measure? • Different colour spaces can be used to define a colour. Please note that the colour observation is always dependent of illuminant and observer (e.g. D65/10°) The colour – CIELab • The CIE L*a*b* colour space is the most used in coil coating industry • It defines the colour coordinates L*, a* and b* L* represents the colour lightness a* represents the colour green-red dimension b* represents the colour blue-yellow dimension Colour characterisation The colour – CIELCh • CIELCh uses the same colour space than CIELab, defining it with three colour coordinates L*, C* and h* L* represents the colour lightness C* = represents Chroma or saturation of the colour. Its value can be obtained from CIELab by C* = ͕∗ͦ + ͖∗ͦ h* axis represents hue ranging from 0 (grey) in the centre of the circle to 100 (saturated) around the edge. All possible saturated colours are present around the edge of the circle. The units are expressed as degrees from 0 ° (red) through 90° (yellow), 180° (green), and 270° (blue) back to 0°. Colour characterisation Colour measurement – Diffuse geometry differences of geometry a) 8°/D (0 °/D) b) D/8° (D/0°) Multiangle Directional geometry c) 45/0° d) 0°/45 Colour characterisation Colour measurement - spectrophotometry • Different types of equipment Spectrophotometer available X-Rite: 964, 45/0° • Colorimeter, spectrophotometer, Spectrophotometer spectroradiometer X-Rite: VS450, 45/0° Colorimeter Spectrophotometer Spectrophotometer Minolta: CR 400, D/0 ° HunterLab: UltraScan, D/8 ° BYK Gardner: BYK-mac, multiangle Colour characterisation Colour measurement – differences of measurement • Depending on the equipment used • Difference between the producers for the same kind of equipment • More important with fabrication time • Difference between one equipment family and another explained e.g. by different parameters • sphere diameter, light source (tungsten, xenon, diode, etc.), pulsed or continuous light, etc. • Depending on the geometry used • L*a*b* absolute values depend on the geometry used • Always small dispersion in absolute value due to equipment calibration • Mandatory to observe all the equipment conditions before concluding a difference of colour Colour characterisation Metamerism • In some cases two prepainted samples can look exactly the same under one light source, but when transferred under a different light source, their visual appearance differs significantly. • This is called metamerism. Colour characterisation smooth orange peel Appearance • The final appearance of prepainted metal depends also on the type of resin grained structured used, film thickness, gloss, and the surface texture (structured, granulated, embossed, metallic,…) embossed wrinkled Colour management Colour management Colour difference ∆E* • The colour difference between a reference and a sample is determined by the deviation on the different axes: ∆L*, ∆a*, ∆b* • ∆E* (colour deviation) = ∆͆∗ͦ + ∆͕∗² + ∆͖∗² • Visual evaluation is necessary because ∆E* criteria can be insufficient (metallized colours, colours with high whiteness, limit of the equipment sensitivity, non-uniformity of CIELab colour space…) • Colour difference is always measured from 2 physical samples (never with absolute values as references) • It is practically impossible to have a ∆E* 0.0 colourmatch, a small difference of ∆E* is normal and expected. Colour characterisation The colour - CMC • CMC is a modification of ΔE of CIELab developed by the Color Measurement Committee of the Society of Dyers and Colorists. It is defined in AATCC Test Method 173, “CMC: Calculation of Small Color Differences for Acceptability.” • The CMC calculation mathematically defines an ellipsoid around a defined and agreed standard colour. The dimensions of the ellipsoid depends on the colour shade and thus can be expected to better correspond to the sensibility of the human eye. • This ellipsoid consists of a semi-axis that corresponds to the attributes of hue (h), chroma (C), and lightness (L) • It represents the area of acceptance in relation to the standard, the same way the CIELab "box" defines acceptable difference limits • The size of the ellipsoid varies depending on its position in the colour space and its shape is influenced by the CMC ratio l/c lightness/chroma. The c (chroma) tolerance is usually smaller, since human eye is able to detect smaller shifts in chroma than in lightness. Also, in the orange region, ellipsoids are narrower, while in the green region, ellipsoids are wider. Colour management Colour category definition C* ≤ 10 Category 1 10 < C* ≤ 20 Category 2 • Colour classification by category is L* > 80 20 < C* ≤ 30 Category 3 obtained from the colour C* > 30 Category 4 parameters L*, a*, b*, C* C* ≤ 10 Category 2 C* ≤ 25 and -11 < a* < 11 Category 2 and -5 < b* < 25 60 < L* ≤ 80 - Category 1: light colours C* ≤ 30 - Category 2: medium colours and -16 < a* < 16 Category 3 and -5 < b* < 25 - Category 3: dark colours C* > 30 Category 4 - Category 4: sharp/saturated colours C* ≤ 30 Category 3 L* ≤ 60 - Category 5: metallized colours C* > 30 Category 4 NOTE! Colours that do not fall into any category are classified by the paint supplier. Colour management Examples of colour categories Colour difference tolerances - recommendations Colour tolerance recommendations General tolerances to be applied using 45/0°, 0°/45 and D/8° geometries with spectro equipment Geometry used 45/0° or 0°/45 Colour tolerance recommendations DE CMC D/8° CIELab CF = 1,00 L:C= 2,00 Category 1: light colours ΔE* ≤ 1,0 ΔE* ≤ 1,0 Category 2: medium colours ΔE* ≤ 1,3 ΔE ≤ 1,0 Category 3: dark colours ΔE* ≤ 1,5* CMC ΔE* ≤ 1,5* Category 4: sharp/saturated colours ΔE* ≤ 1,5* Category 5: metallized colours visual control required * ΔE* ≤ 2,0 can be accepted for specific colours Note! The supplier has the final responsibility of defining the colour category of his products. This applies particularly for special, demanding colours and cases where the colour is close to a limit between two categories, or where a colour shade falls out of any category Colour tolerance recommendations • Metallized colours: Interaction light-material • ΔE* measurement can be disturbed by interaction between the incoming light and the surface of the metallic flake. Visual control remains important. • Specular reflection on metallic fillers: mirror effect, variable effect on the reading depending on the orientation of the angle Colour tolerance recommendations Guidelines to ensure colour consistency • Different production batches should not be used next to each other even if they are of the same colour AND even though all coils from different orders have the same tolerance compared to the colour master standard. • ΔE* guaranteed depends on the colour category Examples of colour differences ∆b=+5 ∆a=+5 ∆L=-5 ∆L=+5 ECCA Green ∆a=-5 ∆b=-5 ∆b=+5 ∆a=+5 ∆L=-5 ∆L=+5 ECCA 50% Grey ∆a=-5 ∆b=-5 ∆b=+5 ∆a=+5 ∆L=-5 ∆L=+5 RAL 7035 Light grey ∆a=-5 ∆b=-5 ∆b=+5 ∆a=+5 ∆L=-5 ∆L=+5 RAL 1015 Light ivory ∆a=-5 ∆b=-5 ∆b=+5 ∆a=+5 ∆b=+1 ∆a=+1 ∆L=-5 ∆L=+5 RAL 7000 Squirrel Grey ∆L=-1 ∆L=+1 ∆a=-5 ∆b=-5 ∆a=-1 ∆b=-1 Colour matching and specification Colour match and specification Procedure • The colour master is the colour matched by the paint supplier • No colour match is based on theoretical values • A physical colour match is the closest proposition to the colour master: the chemical paint composition is perfectly fixed
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